Dropbox drops onto the iPhone
The iPhone app works with Dropbox's photo gallery feature, so it's easy to upload and view photos on the go. You can also download any of your stored files that you can view on an iPhone, which strikes me as pretty darn brilliant: now, instead of just using Dropbox to back up your computer, you can use it to back up all the media on your phone, too.
If you're away from a computer and your iPhone's music and photos get wiped, they'll still be safely waiting for you in your Dropbox.
Update: Although you can obviously store music (and anything else) on your Dropbox account, there's not actually a way to sync music to your iPhone via Dropbox. Oops. Photos and videos definitely work, though.